The present invention relates to an acceleration detecting apparatus formed by a semiconductor effectively capable of executing an electronic suspension control by detecting, for example, the acceleration state or the fluctuating motion of an automobile, being able to detect the acceleration or the vibration, for instance, in three dimensions by means of one semiconductor element.
When detecting a moving object, for example, in three dimensions, three detectors for detecting the accelerations of the object in three different dimensions of X-, Y- and Z-axis directions are necessary, each corresponding to the respective dimension.
FIG. 11 shows an example of the construction of a conventional 3-dimensional acceleration detecting apparatus heretofore considered. In this apparatus, triangular pyramid mount 52 is installed on base 51. The surfaces of mount 52 are formed in the directions of the three dimensions. Beam-shaped vibration plates 53, 54 and 55 are so mounted as to extend toward perpendicular directions from the respective surfaces of mount 52. Masses 56 to 58 are respectively attached to the freely vibrating ends of plates 53 to 55, and the plates 53 to 55 are vibrated by the vibration in X-, Y- and Z-axis directions of three dimensions.
Strain gauges are respectively mounted on plates 53 to 55 to convert the vibrating states of plates 53 to 55 to electric signals and to output the signals.
However, in such an acceleration detecting apparatus of the constructed described above, vibration plates 53 to 55 are mounted to extend in different directions, and the angular relationship of the extending directions of plates 53 to 55 must be accurately set. Therefore, the detecting apparatus must have a complicated construction and the operational characteristics must be regulated substantially in a manner similar to a manual procedure. This apparatus is indispensably difficult to be reduced in size, and is difficult to be, for example, interfaced with a microcomputer for executing suspension control of an automobile.
A detector for detecting an acceleration of one dimension in a simple case is composed of a vibration plate to be used in an apparatus as shown in FIG. 11, and a detector formed of semiconductor is considered to reduce its size. More particularly, a beam formed, for example, of polysilicon is cantilevered over a semiconductor so that a small interval is formed between the beam surface and the surface of the semiconductor substrate. When the beam is vibrated, an electrostatic capacity between the beam and the surface of the substrate varies, and the beam vibrating state is detected by detecting the variation in the capacity.
However, it is very difficult to construct a 3-dimensional acceleration detecting apparatus for detectors with the above-mentioned construction. If the beam is warped due to a temperature change, the interval between the beam and the surface of the substrate alters, causing the electrostatic capacity therebetween to change. Therefore, this apparatus cannot accurately and stably detect the acceleration.
It is in this case possible to set intrinsic resonance vibration frequencies to the respective beams by differentiating the lengths of the beams by combining several beams of the above-mentioned construction and to construct a vibrating spectrum detecting apparatus. However, since the interval between the beams and the surface of the substrate is small, if dust is introduced into the interval, it not only affects the capacity but also varies the resonance frequency of the beam, causing the vibration detecting operation to become unstable.
In addition, as disclosed, for example, in a thesis of LYNN MICHELLE ROYLANCE of IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL., ED-26, NO. 12, DECEMBER 1979, a construction for holding a vibration beam formed of silicon between a pair of glass covers in a sandwich shape is considered. However, it is difficult to construct a small-sized 3-dimensional acceleration detecting apparatus of such a construction.